1. Field of the Invention
The present invention relates to a method of manufacturing a Hg-series 1223 type oxide superconductor which exhibits superconductive characteristics at temperatures higher than the liquid nitrogen temperature.
2. Description of the Related Art
A superconductor exhibits various characteristics which cannot be found in other substances. Specifically, the electrical resistance of the superconductor is rendered zero at temperatures lower than a critical temperature Tc, with the result that the superconductor exhibits a complete diamagnetism and produces Josephson effects. Because of these characteristics, the superconductor is expected to be used for forming, for example, a power cable without current loss, a superconducting generator, a material for high field magnets for nuclear fusion plasma confinement, a material for magnetic levitation vehicle, a magnetic shielding material, and a high speed computer.
Since Bednorz and Muller found in 1986 a La--Ba--Cu--O series superconductor having a Tc of about 30K, it is reported that superconductivity has been recognized in about 30 kinds of copper oxides. Particularly, a Tl--Ba--Ca--Cu--O series superconductor having a Tc of 125K has attracted attention as a superconductor exhibiting the highest Tc. On the other hand, it has been reported recently that a Hg--Ba--Ca--Cu--O series superconductor (1223 type) exhibits a Tc of 133.5K (A. Schiling et al., Nature (London) 363,56 (1993)).
The Hg--Ba--Ca--Cu--O series oxide superconductor of substantially single phase ceramics can be produced by a synthetic method, which necessitates an ultra-high pressure of 5 to 6 GPa (H. Hirabayashi et al., Jpn. J. Appl. Phys. 32, L1206 (1993)). The synthetic method necessitating such an ultra-high pressure requires an ultra-high pressure synthetic apparatus which is highly costly and, thus, is not adapted for mass production of the oxide superconductor. In addition, the particular synthetic method permits producing an oxide superconductor in only such a small amount as about 10 mg in a single operation. Further, the obtained oxide superconductor does not have a single phase completely.
Also known is a method of obtaining a Hg series oxide superconductor without requiring an ultra-high pressure. In this method, a powder mixture is prepared by mixing HgO with a complex oxide such as a Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.7 complex oxide, or a complex oxide of BaCuO.sub.2 and Ca.sub.2 CuO.sub.3, followed by shaping the powder mixture and subsequently applying a heat treatment to the shaped body at 800.degree. to 850.degree. C. for 5 to 10 hours. However, the resultant Hg series oxide superconductor contains many impurity phases, leading to a low volume fraction, i.e., less than 90% and a Meissner signal less than 10%.